The operative word is "radically." Things aren't radically different. However, let's actually examine your examples...
1. Affordable Healthcare. Did you know that the bill originated from many Republicans? Many Republicans actually supported a similar bill in the early 1990's. However, the same representative
s and senators now oppose "Obamacare." Why? Political games.
2. DOMA. DOMA was probably never going to pass, and Republicans knew that. Even if it did, they know the Supreme Court will over-rule it. They are a multi-billion dollar organization. They got someone punching the numbers and doing polls. So, if the Republicans know that it will never work, why continue to fight for it? It could be about principle, but more likely, they make it an issue so the issue can be a rally for for people to side with the party. Republicans know people fear/dislike homosexuality, so they use it for votes. How many times have you heard phrases like "the defense for traditional marriage" or "the sanctity of marriage" or "It's Adam and Eve not Adam and Steve." These phrases are ingrained into the public for a reason. Look at Rob Portman. He conveniently backs gay rights now that it actually affects his family. Even then, it took him 2 years. Political games.
3. War with Russia. Our situation with Russia probably would be similar. No president would want to go to war with Russia. Chances are that they still have nukes. And historically, fighting Russia is a bad idea. The rhetoric might be different, but the situation would be similar. People who are insulting Obama for being "weak" are only doing so because they are playing political games. Obama looking bad is good for the Republicans. Look at the last election.
Having some influence and creating radical change are not the same thing. There are many things that are different, but there are many things that aren't. As much as people are arguing and fighting about politics, this isn't a civil war.
BTW, both parties want people to argue among each other. It's free publicity, and it creates stronger supporters. Look at Believe_N_Me. She is not even arguing about issues anymore; she's arguing against people. Politicians like that. If people actually focused on the real issues and real progress, politicians wouldn't make any money. Recently, politicians got paid for getting nothing done. Congress "shut down." Congress even got a raise; Republicans and Democrats included. How do you get a raise for being so inept? Political games.
It’s just not liberals and conservatives, its religious people, its poor people, it’s almost any group you can find out there. People share common interest and believe the only way they can change their surroundings is to push it together. There will be many times where the group is exploited by others, but as a mass groups tend to be stupid and irrational anyways.
But regarding your statement about whether control by one party influences things radically, I would have to disagree to a certain extent. Would something like Affordable Healthcare been pasted if the Republicans controlled Congress and the presidency? Would DOMA have been struck down? Would we have re-established relations with Cuba? Would we have been at War with Russia already? To a certain degree certain things will remain unchanged or result in the same outcome regardless of political parties, but I think which group holds power does influence some outcomes.